1. Field of the invention
The present invention relates to an active matrix substrate for constructing a matrix-type display apparatus in combination with a display medium, such as liquid crystal, EL light emission layer or plasma luminosity, and to an active matrix display apparatus using the substrate.
2. Description of the Prior Art
Conventionally, high image-quality matrix display apparatuses have used an active matrix driving system, in which an individual picture element electrode is selected by a switching element so that a plurality of selected picture element electrodes form a display pattern.
FIG. 24 is a plan view of the portion of a conventional active matrix substrate, in which gate bus lines 9 functioning as scanning lines extend in parallel to each other between picture element electrodes 8 and a gate electrode 2 branches from each gate bus line 9. A thin film transistor (to be hereinafter referred to as TFT) 11 is formed as a switching element on the gate electrode 2. Source bus lines 10 perpendicularly intersecting with the gate bus lines 9 and serving as signal lines extend in parallel to each other. A source electrode 7a of the TFT 11 is connected to each source bus line 10 and a drain electrode 7b of the TFT 11 is connected to a picture element electrode 8. Between each gate bus line 9 and each source bus line 10 is formed a gate insulating film formed on the entire surface of the substrate. Between the aforesaid active matrix substrate and an opposite substrate is charged a display medium, such as liquid crystal or the like, resulting in an active matrix display apparatus. Voltage is applied to the display medium between the picture element electrode 8 of the active matrix substrate and an opposite electrode of the opposite substrate, so that the display apparatus performs a display.
Usually, several ten thousands to hundred thousands of picture element electrodes are formed on an active matrix substrate of a display apparatus in order to perform an accurate image display. With the active matrix display apparatus having so many picture element electrodes, picture element defects arise. A factor causing the picture element defects includes poor resist and/or poor etching during the patterning in a TFT formation process, an occurrence of defects on a film in the thin film formation process, mixture of conductive foreign objects into the liquid crystal, and the like. These picture element defects remarkably deteriorate the image quality, causing a low production yield of the display apparatus.
In order to reduce deterioration of image quality when picture element defects occur, it is proposed to use an active matrix substrate shown in FIG. 25. In the active matrix substrate, a picture element electrode 8 is divided into two divided electrodes 20 and 21, which are provided across a gap 23. The divided electrodes 20 and 21 are connected with TFTs 11a and 11b by means of drain electrodes 22a and 22b respectively. Source electrodes 21a and 21b of the TFTs 11a and 11b are connected to the same source bus line 10, a gate electrode 2 common to the TFTs 11a and 11b being connected to a gate bus line 9. Accordingly, the divided electrodes 20 and 21 are simultaneously driven by the same gate bus line 9 and source bus line 10.
FIG. 26 is a sectional view of a display apparatus using the substrate, taken on the line J--J in FIG. 25. On a glass substrate 1 is formed the gate electrode 2 of tantalum (Ta) with a thickness of 2500 .ANG., and on the gate electrode 2 is formed an anodic oxidization film of tantalum oxide (Ta.sub.2 O.sub.5) with a thickness of 3000 .ANG.. A gate insulating film 4 with a thickness of 300 .ANG. made of silicon nitride (SiNx) is coated on the entire surface of anodic oxidization film 3.
On a gate insulating film 4 above the gate electrode 2 is formed a semiconductor layer 5 with a thickness of 1000 .ANG. of intrinsic semiconductor amorphous silicon (to be hereinafter referred to as a-Si (i)). On the semiconductor layer 5 are formed contact layers 6 having a thickness of 500 .ANG. each of n-type semiconductor amorphous silicon (to be hereinafter referred to as a-Si (n.sup.+)). On the contact layers 6 are formed a source electrode 21a and a drain electrode 22a of titanium, the thickness of each of which is 3000 .ANG., resulting in TFT 11a.
On the gate insulating film 4 and drain electrode 22a is patterned the picture element electrode 8 with a thickness of 1000 .ANG. of ITO. Furthermore, on the entire surface of the substrate are formed a protective coat 16 with a thickness of 3000 .ANG. of SiNx and an orientation film 17.
An opposite substrate opposite to the active matrix substrate formed as above-mentioned is provided with a color filter 14 and a black stripe 15 on a glass substrate 12. On the entire surfaces of color filter 14 and black stripe 15 are an opposite electrode 13 and an orientation film 18 of ITO. Between the two orientation films 17 and 18 is charged a liquid crystal layer 19 so as to constitute an active matrix display apparatus.
With the above-mentioned construction, even when the picture element defects occur at one divided electrode, the picture element defects are inconspicuous because the other divided electrode continues to operate. However, on the picture element electrode at which no picture element defects occur, a gap 23 exists between the respective divided electrodes 20 and 21, and no voltage is applied to the liquid crystal layer 19 positioned between the gap 23 and the opposite electrode 13, so that the portion of the liquid crystal corresponding to the gap 23 does not contribute to the display. Accordingly, another problem is created as follows: A normally white mode liquid crystal apparatus, passes light through the gap 23 even when voltage is applied, so the contrast decreases. A normally black mode liquid crystal display apparatus does not pass light through the gap 23 even when voltage is applied, the whole display picture plane becomes dim. With the display apparatus using plasma or EL luminosity, the amount of emitting light per a unit area decreases which makes the whole display picture plane dim.
To solve the above-mentioned problem, an active matrix substrate as shown in FIGS. 27 and 29 can be proposed. FIG. 28 is a sectional view taken on the line C--C in FIG. 27, and FIG. 30 is a sectional view taken on the line E--E in FIG. 29. The display apparatus shown in FIGS. 27 and 29 are similar to that in FIG. 25, but different therefrom in that a superposed region 57 is provided at a region where divided electrodes 20 and 21 are adjacent to each other. These display apparatus, which are provided with the superposed regions 57 respectively, can contribute to a display even in the area positioned between the divided electrodes 20 and 21. In other words, since the divided electrodes 20 and 21 are superposed so as to sandwich a gate insulating film 28 therebetween in the superposed regions 57 shown in FIGS. 27 and 28 respectively, voltage can be applied to liquid crystal layers 41 positioned between the regions 57 and the opposite electrodes 39, respectively. The display apparatus shown in FIGS. 29 and 30 has divided electrodes 20 and 21, each of which forms a two-layer construction, and in each superposed region 57 are superposed the upper layer divided electrode 20b and the lower layer divided electrode 21a to each other. Accordingly, in the respective display apparatus, voltage can be applied to the liquid crystal layer 41 between the superposed region 57 and the opposite electrode 39, as well. The above-mentioned display apparatuses shown in FIGS. 27 and 29, in which the region that does not contribute to a display does not exist between the divided electrodes 20 and 21, do not cause the lowering of the contrast or brightness at the display picture plane.
However, even in these display apparatuses, once a TFT connected to the divided electrode malfunctions, deterioration of image quality caused by the occurrence of picture element defects cannot be avoided. To avoid the occurrence of such picture element defects, a correction technique using laser beams has been developed. For example, Japanese Laid-Open Patent Publication No. 59-101693 discloses that, when a TFT malfunctions, a laser beam is used to disconnect the defective TFT from the gate bus line and source bus line, so that the defective picture element electrode is connected to the adjacent picture element electrode, thereby restoring the TFT, the connection being carried out by irradiating a laser beam to a prepared restorable construction, which comprises an electrically conductive film provided across the two adjacent picture element electrodes. The electrically conductive film contacts one picture element electrode and is superposed on the other picture element electrode in a non-conductive state. A laser beam is irradiated so as to melt the picture element electrode and electrically conductive film superposed to each other in the non-conductive state, and to electrically connect between the picture element electrode and the electrically conductive film, thereby carrying out the restoration of TFT. Such a restoration will avoid the malfunction of TFT even though picture element defects arise, but the restored picture element electrode operates in the same way as that of the adjacent picture element electrode, and its inherent display operation cannot be performed.
Another construction for correcting picture element defects is disclosed in, for example, Japanese Laid-Open Patent Publication No. 61-153619, in which a plurality of switching elements are provided per one picture element electrode, and one of the switching elements is connected to the picture element electrode and the other is not connected thereto. When the switching element connected to the picture element electrode becomes defective, a laser trimmer, an ultrasonic cutter or the like is used to disconnect the defective switching element from the picture element electrode, and another switching element is connected thereto. The switching element is connected to the picture element electrode by bonding a minute conductor thereto by means of a dispenser or the like, or by coating Au, Al or the like onto a predetermined portion of the substrate. Moreover, Japanese Laid-Open Patent Publication No. 61-56382 discloses a construction in that a laser beam is irradiated onto the superposed portion of two metal layers, and the metal layers are melted so as to be electrically connected to each other.
The above-mentioned defect correction must be carried out on an active matrix substrate before the display apparatus is assembled. The reason for this is that, after completion of display apparatus, part of metal evaporated or melted by the laser beam irradiation enters into a display medium, such as liquid crystal, interposed between the picture element electrode and the opposite electrode, thereby remarkably deteriorating the optical characteristics of the display medium. Accordingly, the correction of picture element defects is always carried out at the active matrix substrate manufacturing process prior to the assembly of display apparatus, in other words, prior to the charge of display medium.
However, it is very difficult to detect picture element defects in the state of active matrix substrate. Especially, for a large-sized display apparatus consisting of picture elements of one hundred thousand to five hundred thousands or more, in order to find a defective switching element, by detecting the electrical characteristics of all the picture element electrodes, measuring instruments of extremely high accuracy must be used. Therefore, the detection process becomes complicated and mass-production becomes difficult, resulting in an expensive display apparatus. For the above-mentioned reasons, a large-sized display apparatus of large number of picture elements, in fact, cannot attain the correction of picture element defects in the state of substrate by the use of laser beams.
With the display apparatus for performing a high density display, the entire length of bus wirings connected to switching elements are extremely long. Therefore, a defective connection and/or a defective insulation is liable to occur in the bus wiring, which especially occur in the region where the bus wirings intersect with each other. The occurrence of such a defect is visibly recognized as a line defect starting from a picture element in the vicinity of the defective portion, the line defect remarkably deteriorating the image quality to create a serious problem in the manufacture of display apparatuses. Techniques to correct such a defect must be developed.